The present invention relates to a method of spreading particles and a spreading apparatus which are used to spread particles between two substrates for a liquid crystal display panel, such that the particles serve as spacers for controlling the distance between the substrates.
In recent years, demands have arisen for a liquid crystal display panel with high performance such as high contrast, a wide view angle, etc., and with high display quality enough to create a uniform display image over the entire display element, without defective display images.
In general, a liquid crystal display panel is constructed in a manner in which two substrates are arranged so as to face each other and liquid crystal is enclosed between these substrates. In this case, it is very important for both the high performance and high display quality to control the distance between the two opposite substrates to be uniform over the entire areas of the substrates. Specifically, it is necessary to control the distance between the substrates to be maintained at a predetermined value, in order to achieve high performance of the liquid crystal display element, and it is also necessary to keep the distance between the substrates to be uniform over the entire areas of the substrates, in order to further achieve high display quality.
Hence, in order to obtain a uniform substrate distance over the entire areas of the substrates, a method has been adopted in which spacers consisting of particles each having a desired diameter are uniformly spread on the surface of one of the substrates and the other substrate is thereafter adhered thereon.
For example, Japanese Patent Application KOKAI Publication No. 6-3679 discloses an apparatus for spreading spacers, comprising a swingable spreading nozzle for spreading spacers and a movement mechanism for relatively moving the spreading nozzle and a substrate in the X- and Y-directions in a horizontal plane. In this apparatus, spacers are spread onto the substrate from the nozzle, while relatively moving the spreading nozzle and the substrate such that an extended line of the center axis of the spreading nozzle draws a saw-tooth-like zigzag trace on the substrate.
However, according to such a conventional spreading apparatus and a method thereof as described above, the distance between parts of the trace including folded portions is smaller at each folded portion, so that the spread areas of particles on the substrate are overlapped at the folded portions of the trace. In these overlapped areas, the amount of the spacers thus supplied is larger and the spread density is higher than in the other areas. For example, in case where spacers each having a diameter of about 5 .mu.m is spread at a spread density of about 150 particles/mm.sup.2, the spread density is increased to 150 to 170 particles/mm.sup.2 in the overlapped areas. Therefore, the spread density of the spacers on the substrate is thus not uniform so that it is difficult to maintain a uniform distance between two substrates.
Note that the trace of spread particles means the trace of movement of the center of distribution of spread particles, i.e., the trace of a cross point between an extended line of the center axis of the spreading nozzle for spreading particles and the surface of the substrate.
In order to prevent the spread density from being not uniform, the swing angle of the spreading nozzle may be increased so as to make folded portions of the trace be positioned outside the substrate. However, the use amount of spacers will then be increased and the time required for spreading will be elongated.
Meanwhile, in a method of spreading spacers through a spreading nozzle, the spacers are electrically charged by friction caused as the spacers transferred through a convey path, and are thereby applied with positive or negative electric charges. This kind of charging of the spacers may be preferable in view of preventing the spacers from being coagulated. If the spacers thus electrically charged are spread onto the substrate, a number of lines such as scanning lines, signal lines, and a short ring formed on the substrate are also electrically charged to a potential equal to the spacers.
However, since complicated electronic circuits are provided on the substrate of a liquid crystal panel, the substrate is not uniformly charged over its entire region, but only several particular lines or the likes are charged. In particular, when spacers are spread while moving the spreading nozzle so as to draw a certain trace, particular lines or the like on that region of the substrate where spacers are not yet spread are charged to a potential equal to the potential of the spacers, and those lines or the like repulses spacers which are spread later on that region. Therefore, the spread density of the spacers on such a region of the substrate is decreased abnormally so that non-uniformity occurs.
In addition, in case where the substrate is uniformly charged by any method, electric charges are gradually applied to the lines and the like of the substrate as spacers are gradually spread on the substrate. Thus, charging of the substrate leads to occurrence of defects such as electrostatic breakdown or the like of the electronic circuit and is therefore not desirable.